A Traveling-Wave Tube (TWT) may act as an amplifier or an oscillator for Radio Frequencies (RF). This is accomplished through the interaction of an electron beam and an RF circuit known as a slow wave structure, where the RF wave velocity as it travels down the circuit is much less than that of light in a vacuum. As the electron beam travels down this interaction region, an energy exchange takes place between the electrons and the RF circuit wave. When a traveling wave tube is configured as an amplifier, RF energy is applied to an input port, and the interaction between the RF and the electron beam produces power gain, and the amplified signal is removed from an output port. When a traveling wave tube as an oscillator, at some frequency there is sufficient internal RF coupling through the gain element at a particular frequency to enable oscillation at that frequency. Backward wave devices have the property that this oscillation frequency can be controlled by the voltage applied between the cathode and anode of the electron gun.
FIG. 1 shows the three basic components to any TWT or linear beam device. A TWT includes an electron gun which has a thermionic or field emission cathode 108, a slow wave circuit shown as input coupler 116, output couplers shown as backward wave couplers 118 and 120, and a collector shown as 112. The electron gun emits electrons and the application of a high differential voltage optionally combined with a magnetic focusing circuit (not shown), the electrons travel down electron beam 114 tunnel terminating in collector 112. The voltage applied to the cathode may range in value from several hundred to several hundreds of thousands of volts. The slow wave structure 116 which is shown generically coupled to electron beam 114 may couple RF energy into the electron beam 114, or it may provide a source of oscillation coupled to electron beam 114, or it may act as an amplifier whereby it includes an input port (not shown) and has the characteristic of a bandpass filter for RF waves in the region of interest. Over a particular band of frequencies, which can range as high as two or more octaves, the slow wave structures 118 and 120 may provide a frequency transfer function for the RF energy traveling through them. There are numerous types of slow wave structures including helical, coupled-cavity, and ring-and-bar circuits. The frequency at which the device operates is determined by the geometry and size of the slow wave structures 116, 118, and 120. In a backward wave device, the slow wave structures 118 and 120 cause RF energy in the circuit to counter-propagate, or propagate toward the electron gun to an output port, as will be explained later. After the RF energy has been coupled into and extracted from the electron beam using slow wave structures such as backward wave couplers 118 and 120, the beam enters a region known as the collector 112, which collects the spent beam. There are many collector configurations used in linear beam devices. Some of these include single-stage grounded collectors and multiple stage collectors. The driving concept behind the selection of collector used is efficiency and power supply considerations.
A backward wave device, whether it be an amplifier or an oscillator, is a type of traveling wave device which includes a slow wave structure which causes the phase velocity of a forward moving wave to have a negative value, so that it travels in a direction counter-propagating (opposite the direction of) the electron beam 114.
FIG. 2 shows a ω-β curve for an electron beam interacting with a slow wave structure such as backward wave coupler 120 of FIG. 1, where the x axis 105 is the wave number, which for corrugated structures are normalized to k*d, where:                k is the wave number, or 1/λ, and λ is the wavelength of interest;        d is the depth 123 of the corrugations shown in FIG. 1;        and the period of pitch p 121 of FIG. 1 is constant. The y axis of the graph shows the upper cutoff frequency, for a structure, where        fcutoff is proportional to 1/d*c        where        d=depth of corrugation, as before,        c=velocity of light.Curve 102 is the electron beam line, the slope of which indicates the electron beam velocity as electrons leave the cathode and travel down the beam tunnel, and the slope of this line 102 increases with larger voltage applied by cathode 108 in FIG. 1. The functional characteristics of a slow wave structure having a fixed pitch p 121 from FIG. 1 and varying depth d 123 from FIG. 1 is shown as curve 106a, 106b, and 106c, which for corrugation structures is governed by the parameters p 121 and d 123 both from FIG. 1. Smaller values of d yield a higher cutoff frequency, and larger values of d result in a lower cutoff frequency. Operation of the RF slow wave structure with a large cathode electron acceleration voltage results in an intersection point between the electron beam line 102 and the slow wave structure curve 106a, 106b, or 106c in the region 0 to n, and the device operates as a forward wave device. A reduction of the cathode electron acceleration voltage results in a lower slope of the electron beam line 102, and the electron beam line 102 intersects the RF slow wave structure characteristic curve at point 104. Operating point 104 is shown in the region from n to 2n known as the backward wave region, and the RF waves are counter-propagating with the electron beam, where the RF is propagating in a direction opposite the direction of the electron beam. For a given slow wave structure geometry, as the electron beam voltage is slightly increased, curve 102 has a greater slope, and intersection point 104 supports at a higher operating frequency F1 101. For given operating point 104, traveling waves can be supported up to a frequency F1 101 where the corrugation depth d=80u, as shown in the present example. If the traveling waves experience a change in corrugation depth to 100u as shown in characteristic curve 106c, the slow wave structure will no longer support traveling waves at this frequency, and the waves will be reflected in the region of the discontinuous interface where the depth d in increased. The curves 106a, 106b, and 106c are normalized to wave number in the x axis and show the relationship between corrugation depth and the maximum RF frequency the slow structure can support. The curves of FIG. 2 are ordinarily computed using numerical techniques for a specific structure. In the present example, curves of FIG. 2 were calculated for the case where the corrugation pitch p=50u and the width of the individual structures is 20u for a variety of depths d 123 (from FIG. 1) ranging from 40u to 100u. These curves, in conjunction with the electron beam line 102 enable the design of reflecting structures for use in forward or backward wave regions. One of the problems with devices that operate in backward wave regions is the inefficiency of coupling between the slow wave structure and the output waveguide.        
FIG. 3 shows a backward wave structure from the unpublished design of a Russian-designed microwave tube available commercially in Russia. An electron beam 135 travels from a beam tunnel entrance 130 through a beam shaper 132 to a beam tunnel exit 138, and beam shaper 132 is at the same height as corrugations 136 having a depth d in accordance with the characteristics of FIGS. 1 and 2. Additionally, the beam shaper includes a series of slots parallel to the electron beam 135 axis which cause the electron beam 135 to travel over and around the corrugations which are perpendicular to the electron beam 135. This dual corrugation produces pin structures known as pintles 136 which have a depth d and pitch p perpendicular to the axis of the electron beam 135. These pintles 136 include longitudinal slots which allow the electron beam to surround the pintles 136, and therefore interact with the them in an enhanced manner. Section z—z through the beam shaper 132 of FIG. 3 is shown as FIG. 3a showing the slots in the beam shaper 132 and the electron beam 135 forming around these slots. These slots continue in the pintles 136 shown in section view a—a in FIG. 3a with electron beam 135. The cross section through pintles 136 of section b—b is shown in FIG. 3b, which effectively shows a top view of the pintles 136 and also pintles 134 from the sloping region of FIG. 3. The pintles 136 are physically small and not well thermally coupled to substrate 131 in FIG. 3, and an imperfectly aligned electron beam 135 directly impinging on these pintles would cause them to overheat and melt. By machining the beam shaper 132 to the same height as the pintles 136, and including slots in beam shaper 132 which continue through pintles 134 and 136, the shaper 132 is able to very closely couple the electron beam 135 with the pintles, tightly coupling the tops and sides of the pintles 136 with the electron beam 135 as shown in FIG. 3a. The pintles are therefore shielded from overheating due to direct exposure to a misaligned electron beam by the beam shaper 132, which conducts excess heat into the slow wave structure body 131 from FIG. 3. The operation of the backward wave coupler of FIG. 3 includes the reflection of RF energy carried in the beam by sloping structure 134, whereby reflected wave energy is coupled into the output aperture 140. In the unpublished RF device of FIG. 3, the output port 140 is placed between a row of pintles in the sloped region 134. Fabrication of the device shown in FIG. 3 for use in sub-millimeter wavelengths is very difficult, as the features are on the order of 10s of microns, and the sloping section 134 must be completed prior to the pintle fabrication. The best method for pintle feature manufacturing is electro-discharge machining, which is best done using substantially planar surfaces, as opposed to the sloping surface 134.
In prior art devices such as in U.S. Pat. No. 4,263,566 by Guenard and shown in FIG. 1 structures 118 and 120, the slow wave structures are corrugated in one dimension only such that the cross section of FIG. 1 is correct for any section through the slow wave structure. Similarly, the slow wave structure described in U.S. Pat. No. 4,149,107 by Guenard comprises 1-dimensional slots as shown. In the Russian device of FIG. 3, the corrugations perpendicular to the electron beam are supplemented by slots parallel to the electron beam which produce structures referred to as pintles, which are a plurality of pins spaced on regular intervals, typically 10–20 pintles per wavelength, in accordance with the desired frequency performance as described in FIG. 2. While backward wave devices enable operation over a wide range of frequencies tunable by changing the electron beam voltage, backward wave devices suffer from inefficient coupling of RF energy to the output port and the use of pintles increases the efficiency of this coupling.